Electrically powered wheelchairs are well known in the art and generally include two electric motors. One motor is connected for driving the left large diameter rear wheel of the wheelchair and the other connected to drive the right large rear wheel of the wheelchair. By providing an appropriate switching arrangement which can be operated by a patient in the wheelchair, the chair can be driven directly forwardly by energizing both motors simultaneously. Turning to the right or left is accomplished by, respectively energizing the left wheel motor or the right wheel motor. Rearward movement of the wheelchair is accomplished by reversing the polarity of battery energy to the motors to rotate them in an opposite direction to drive the chair backwards. In place rotation or swivelling of the wheelchair can be accomplished by driving the left motor in one direction and the right motor in an opposite direction.
Preferably, the left and right wheel drive motors constitute permanent magnet d.c. motors. However, these motors have the characteristic of operating as a generator when the wheelchair is coasting thereby sending current back through the various control switches and oftentimes burning them out. To avoid this problem, wheelchairs have been designed with motors utilizing a field winding rather than a permanent magnet. When the motor is de-energized, the magnetic field generated by the field winding is also removed so that the problem of the motors acting as generators is avoided. On the other hand, upon de-energizing the motors, the field generated by the field winding collapses and gives rise to large reverse voltage spikes which in turn can damage the sensitive micro switches utilized in the switching system.
Further difficulties are also experienced with presently available electrically driven wheelchairs whether permanent magnet type or field wound motors are used. For example, in executing a right hand turn, the operator would normally energize only the left drive wheel motor. The right drive wheel motor would then simply coast and unless some braking friction were applied to the right wheel, controlling of the turning is somewhat difficult. Similar difficulties are encountered for the same reasons when a left turn is being made. Finally, aside from the problem of the motors acting as generators when the wheelchair is coasting, the coasting condition itself can be dangerous if the only means for slowing down the wheelchair is the throwing of the motors in reverse.